1. Field of the Invention
The present invention relates to a method and apparatus for receiving and/or reproducing digital signals of movies, programs, and the like transmitted via transmission media such as coaxial cables, telephone lines, and broadcast satellites. More particularly, the invention relates to such a method and apparatus capable of constraining a copy of a recorded digital signal.
2. Description of the Related Art
A method of inhibiting a copy of a video signal with a recording/reproducing apparatus is disclosed, for example, in JP-A-61-288582. The technique described in this JP-A-61-288582 adds a signal immediately after a synchronization signal of a video signal so that although a television can display this video signal, a video tape recorder (VTR) can record only an image of poor quality.
Another JP-A-4-360068 discloses techniques of restricting a user to copy data or inhibiting a user to see an image of data, with a data recording/reproducing apparatus.
As a method of compressing digital video signals at a high efficiency, the ITU-T Draft Rec. H. 262 standard called MPEG-2 (Moving Picture Experts Group) is known. MPEG-2 Systems Working Draft is also known which is the transmission standard of video and audio signals compressed by MPEG-2.
The above standards show the techniques of compressing a program and broadcasting it in a digital format. This compression method realizes a large compression rate so that a single transmission channel can broadcast programs four to eight times as many as a conventional analog broadcast. With this technique, services called near video-on-demand are already available in which the same 2-hour movie is repetitively broadcast at an interval of 30 minutes. However, it is impossible to broadcast all programs 24 hours for near video-on-demand. Therefore, subscribers have a great need for recording a program and reproducing it at a desired time to watch it.
In recording/reproducing a digitally compressed and broadcast program, it can be considered that a received digital signal is decompressed and converted into an analog signal to record it with a conventional analog VTR. However, an analog signal recorded in the analog VTR loses a good signal-to-noise (S/N) ratio of digital signals.
It is therefore desired to digitally record a digital broadcast signal. However, no technique is disclosed as yet to record a digital signal compressed and broadcast, for example, in conformity with the MPEG standard. Generally, error correction is performed to a sufficient degree during recording/reproducing digital signals so that even if copies are made repetitively, the image quality is not lowered. However, on the other hand, it is difficult to protect the rights of a copyright holder if a copy without image quality degradation is permitted. Techniques for protecting the rights of a copyright holder are disclosed in the above cited JP-A-61-288582 for conventional analog VTRs. The other JP-A-4-360068 discloses techniques for restricting a user to copy data or inhibiting a user to see an image of data, with a data recording/reproducing apparatus.
However, techniques of recording digital signals compressed and transmitted in conformity with the MPEG standard and restricting copies of recorded digital signals are not shown at all in the above documents.
Although it is desired to directly record a digital broadcast signal itself, the technique of recording digital signals compressed and transmitted in conformity with the MPEG standard is not disclosed as yet. Generally, error correction is performed to a sufficient degree during recording/reproducing digital signals so that it is advantageous in that the same tape can be repetitively viewed without any image quality degradation. However, on the other hand, if the same tape is allowed to be repetitively viewed without any image quality degradation, a video tape like a cell video tape (a commercial video cassette tape of movies or the like) can be formed easily so that protection of the rights of a copyright holder is difficult. JP-A-61-288582 discloses the technique of protecting the rights of a copyright holder for conventional analog VTRs. JP-A-4-360068 discloses the technique of restricting a user to copy data or inhibiting a user to see an image of data, with a data recording/reproducing apparatus.
However, techniques of recording digital signals compressed and transmitted in conformity with the MPEG standard and restricting copies of recorded digital signals are not shown at all.
It is an object of the present invention to provide a method and apparatus for efficiently recording a signal compressed and broadcast, for example, in conformity with the MPEG standard, and constraining copies of such signals.
It is another object of the present invention to provide a method and apparatus for efficiently recording a signal compressed and broadcast, for example, in conformity with the MPEG standard, and constraining reproduction of such signals.
According to one aspect of the invention achieving the above objects, there is provided a method of recording/reproducing a digital signal in which each of a plurality of programs is bit-compressed; the bit-compressed program is encrypted; the encrypted programs are time divisionally multiplexed; the time divisionally multiplexed programs are modulated with a single carrier; the modulated signal is transmitted; the transmitted signal is received; the received signal is demodulated; at least one program in a packet format is selected from the demodulated signal; the selected program in a packet format is decrypted; a packet signal added with time information indicating an arrival time of each packet included in the decrypted signal in a packet format is inputted, and the packet signal with the time information is recorded, the method comprising the steps of: reproducing the recorded packet signal with the time information; and changing at least one bit of information bits of the time stamp contained in the reproduced packet signal, and outputting digital information with the changed time stamp.
In operation of this method, packet signals with time information are inputted to the recording/reproducing apparatus and recorded at packed intervals in order to improve a recording efficiency. In reproducing packet signals, packet intervals are changed to the original intervals in accordance with detected time information added to the packet signals and thereafter output from the recording/reproducing apparatus. Since packet signals are recorded after packing the intervals therebetween, it is possible to record them at a signal rate slower than the input signal, allowing efficient recording. By using the time information added to the packet signal, packet signals can be changed to have the original packet intervals, and thereafter they are reproduced. If at least one bit of the added time information is changed for the reproduction and reproduced signals are supplied to, and recorded with, another recording/reproducing apparatus, the original intervals between these signals cannot be restored because the added time information is different from the original time information. Accordingly, these signals cannot be decompressed and restored.
According to another aspect of the invention achieving the above objects, there is provided a system in which a plurality of bit-compressed channel programs and their guide information including at least information of a date and time of program reception are transmitted and received, a program is selected from the plurality of received programs, the selected program is recorded and reproduced with a recording/reproducing apparatus, the reproduced program is bit-decompressed to restore and display an original program, the system comprising: means for forming new guide information by selecting guide information and information of date and time of the selected program from the plurality of received programs; means for outputting the selected program and the new guide information to the recording/reproducing apparatus at the same time; means for setting a current time by using information of date and time contained in the guide information of the plurality of received programs; means for comparing the current time with the information of the time and data contained in the reproduced new guide information; and means for inhibiting bit expansion of the reproduced program if the comparison result indicates that a predetermined period has lapsed.
According to the structure of this system, the current time can be correctly and reliably set from the guide information of the plurality of received programs. Since the current time is set from the received information, it cannot be changed by a subscriber. Furthermore, new guide information is formed from received guide information when the selected program is recorded, and the new guide information contains information of date and time of program reception. Therefore, a lapse time from the program reception can be known correctly and reliably by comparing the program reception date and time with the current date and time. If the lapse time exceeds the predetermined time, bit expansion is inhibited to restrict reproduction of a recorded tape.
Other objects, features and advantages of the present invention will become apparent from the following description of embodiments of the invention taken in conjunction with the accompanying drawings.